The invention relates to a helical gear mechanism, and more particularly to a worm gear mechanism. The invention further relates to an electric power steering system, comprising a helical gear mechanism.
A helical gear mechanism of the type in question in form of a worm gear mechanism for use in an electric power steering system is known from DE 101 61 715 A1.
The worm gear mechanism described in DE 101 61 715 A1 is disposed in a housing and comprises a worm and a worm gear. The worm is connected to the drive shaft of an electric drive motor by means of a coupling. In the construction shown there, the worm can be pivoted perpendicularly relative to the axis of rotation and is mounted in a pivot or swivel bearing, whereby the interlocking play can be kept constant during the entire lifetime of the gear mechanism. For this purpose, the worm is prestressed radially and is permanently pushed against the toothing of the worm gear (or of the helical gear). It has been found with such constructions that changes in the direction of rotation or load changes in the gear mechanism, together with existing tooth flank play, can cause the tooth flanks of the helical pinion or of the worm to strike against the tooth flanks of the helical gear or worm gear and cause noise. Nonetheless, the tooth flank play should not be set too low during installation because this can negatively impact the efficiency and wear of the gear mechanism. Moreover, this can worsen the feedback properties of a power steering system, which in turns worsens the driver's steering sensation.
The maximum prestress with which the worm can be pushed into the worm gear is limited as the worm gear mechanism would otherwise jam.
The helical gears or worm gears used in electric steering systems are frequently produced from plastic materials. Over a lifetime, these plastic parts are subject to settling and/or wear. So as to ensure the spring loading of the worms against the worm gear over the entire lifetime, a very high spring-loading force is applied in helical gear mechanisms or worm gear mechanisms known from the general prior art when they are new, this force decreasing over the course of the lifetime because the interlocking play increases. In the new state, this creates increased friction and consequent drawbacks in terms of the efficiency in the gear mechanism.
Reference is also made to DE 10 2008 000 506 A1 with regard to the additional prior art.